1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an In-Plane Switching (IPS) mode LCD device and a method of manufacturing the same, which are capable of increasing an aperture ratio.
2. Background of the Related Art
Recently, flat panel display devices have attracted considerable attention because they have screens with thicknesses of only a few centimeters. Among them, an LCD device is widely utilized for laptop computers, monitors, aircraft, etc. The LCD typically includes a lower substrate, an upper substrate, and a liquid crystal layer interposed between the lower and upper substrates. When a voltage is applied to the liquid crystal layer, arrangement of liquid crystal cells of the liquid crystal layer is changed and transmittance of light according to the changes is adjusted, thereby displaying images.
Hereinafter, a conventional LCD device will be described in detail with reference to the accompanying drawings. FIGS. 1A and 1B are perspective views schematically illustrating a twisted nematic (TN) mode LCD device according to the related art. Specifically, FIG. 1A illustrates a state of the related art TN mode LCD device to which a voltage is not applied, and FIG. 1B illustrates a state of the related art TN mode LCD device to which a voltage is applied.
As shown in FIGS. 1A and 1B, the related art TN mode LCD device includes a first substrate 1, a second substrate 3, and a liquid crystal layer 5 interposed between the first and second substrates 1 and 3. The first substrate 1 further includes a first polarizing plate 7 that is arranged on an outer surface of the first substrate 1 and has a transmission axis oriented to a predetermined direction. The second substrate 3 further includes a second polarizing plate 9 that is arranged on an outer surface of the second substrate 3 and has a transmission axis opposite to that of the first polarizing plate 7. Moreover, although not shown in the drawings, the first substrate 1 includes a pixel electrode and the second substrate 3 includes a common electrode, to thereby generate a vertical directional electric field between the pixel electrode and the common electrode.
Referring to FIG. 1A, when a voltage is not applied, the liquid crystal layer 5 is twisted by 90 degrees between the first substrate 1 and the second substrate 3. Herein, when light 10 enters through the second polarizing plate 9, the light 10 passes through the liquid crystal layer 5. At this time, like molecules of the liquid crystal layer 5 that are twisted by 90 degrees, the light 10 is also twisted by 90 degrees to pass through the first polarizing plate 7, thereby displaying a white image. On the other hand, referring to FIG. 1B, when a voltage is applied, the molecules of the liquid crystal layer 5 are vertically arranged between the first and second substrates I and 3 by the vertical directional electric field between the pixel electrode and the common electrode. Herein, when the light 10 enters through the second polarizing plate 9, the light 10 passes through the liquid crystal layer 5. At this time, since a polarizing direction of the light 10 does not rotate, the light 10 cannot pass through the first polarizing plate 7, thereby displaying a black image.
However, the related art TN mode LCD device has a narrow viewing angle. FIGS. 2A to 2C are schematic views illustrating the narrow viewing angle of the TN mode LCD device according to the related art. Specifically, FIG. 2A illustrates a white displaying state in which a voltage is not applied, FIG. 2B illustrates a black displaying state in which a full voltage is applied, and FIG. 2C illustrates an intermediate displaying state in which an intermediate voltage is applied.
As shown in FIG. 2A, when the voltage is not applied, the molecules of the liquid crystal layer 5 are twisted by a minute angle in the same direction and the incident light (depicted by arrows in the drawing) is displayed as white light in all directions. Then, as shown in FIG. 2B, when the full voltage is applied, the molecules of the liquid crystal layer 5 are arranged in a vertical direction due to the effect of the electric field, and the incident light are not twisted. Thus, the black displaying state is shown. Moreover, as shown in FIG. 2C, when the intermediate voltage is applied, the molecules of the liquid crystal layer 5 are arranged in a slant direction, and the displayed states are different according to the directions of the incident light. In other words, the light entered in the direction from right lower side to the left upper side displays black because the polarizing direction of the light is not changed, whereas the light entered in the direction from the left lower side to the right upper side is displayed as white because the polarizing direction of the light is twisted. As described above, a problem with the related art TN mode LCD device is that the displaying state varies depending on the incident angle of the light, and thus the viewing angle is narrow.
For this reason, methods to widen the viewing angle have been researched and developed. One proposal is an IPS mode using a horizontal directional electric field, a vertical alignment (VA) mode using a vertical arranged layer, and an electrically controlled birefringence (ECB) mode. Other proposals are a multi-domain method of dividing a domain and using a mean value of the arrangement of the molecules of the liquid crystal layer, and a phase compensation method of using a phase difference film to change phase difference according to the change of the viewing angle. Hereinafter a related art IPS mode LCD device will be described.
FIGS. 3A to 3D illustrate the related art IPS mode LCD device. Specifically, FIGS. 3A and 3B are a sectional view and a plan view schematically illustrating the related art IPS mode LCD device to which a voltage is not applied, whereas FIGS. 3C and 3D are a sectional view and a plan view schematically illustrating the related art IPS mode LCD device to which a voltage is applied.
As shown in FIGS. 3A and 3B, the related art IPS mode LCD device includes a first substrate 1, a second substrate 3, and a liquid crystal layer 5 interposed between the first and second substrates 1 and 3. The first substrate 1 further includes a first polarizing plate 7 that is arranged on an outer surface of the first substrate 1 and has a transmission axis oriented to a predetermined direction. The second substrate 3 further includes a second polarizing plate 9 that is arranged on an outer surface of the second substrate 3 and has a transmission axis opposite to that of the first polarizing plate 7. Moreover, the first substrate 1 includes a pixel electrode 2 and a common electrode 4 parallel to each other to thereby generate a horizontal directional electric field between the pixel electrode 2 and the common electrode 4. Referring to FIGS. 3A and 3B, when a voltage is not applied, the liquid crystal layer 5 is arranged between the first and second substrates I and 3, and is approximately parallel to the longitudinal direction of the two electrodes 2 and 4. Herein, when the light 10 (of FIG. 1A) enters through the first polarizing plate 7, the light 10 transmits the liquid crystal layer 5. At this time, since the polarizing direction of the light 10 does not rotate, the light 10 cannot pass through the second polarizing plate 9 having the transmission axis opposite to the transmission axis of the first polarizing plate 7. Thus, a black image is displayed.
As shown in FIGS. 3C and 3D, when a voltage is applied, the liquid crystal layer 5 is arranged differently between the vicinity of the first substrate 1 and the vicinity of the second substrate 3. Specifically, due to the horizontal directional electric field between the pixel electrode 2 and the common electrode 4, part of the liquid crystal layer 5, which is located in the vicinity of the first substrate 1, is perpendicular to a longitudinal direction of the pixel electrode 2 and the common electrode 4. On the other hand, part of the liquid crystal layer 5, which is located in the vicinity of the second substrate 3, is parallel to the longitudinal direction of the pixel electrode 2 and the common electrode 4, like the case when the voltage is not applied because the effect of the electric field is weak. Thus, when the light 10 (of FIG. 1A) enters through the first polarizing plate 7, the light 10 then passes through the liquid crystal layer 5. At this time, like the molecules of the liquid crystal layer 5, the light 10 is twisted and passes through the second polarizing plate 9 opposite to the first polarizing plate 7, thereby displaying a white image. As described above, in the related art IPS mode LCD device, since the molecules of the liquid crystal layer 5 are not erected vertically but switched horizontally, the viewing angle is not changed according to the incident angle of the light 10 even when the intermediate voltage is applied.
However, since the pixel electrode 2 and the common electrode 4 are both arranged on the first substrate 1, the related art IPS mode LCD device has a low transmittance. The low transmittance is caused due to the aperture ratio being decreased as much as widths of the pixel electrode 2 and the common electrode 4. Herein, the widths of the pixel electrode 2 and the common electrode 4 are about 4 μm, which is a minimum width capable of being formed by photolithography. When the photolithography is carried out, there is limitation to reduce the widths of the pixel electrode 2 and the common electrode 4 due to the characteristics of the photolithography.